The propagation conditions of radio waves typically vary constantly in radio systems, such as cellular radio systems, which causes variation to the radio signal as a function of time and place, i.e. fading. The changes occurring in the channel impulse response may result from the physical changes in a medium (for instance owing to the variation of the refractive index in the medium as a function of temperature, pressure and partial pressure of vapour) or from changes in the geometry of a link (the movement of a transmitter or receiver or an obstacle on the link).
One signal fading mode, fast fading, arises from multipath propagation characteristic for the cellular radio environment, wherein a signal propagates along various paths between a transmitter and a receiver. Such a channel is referred to as a Rayleigh fading channel (including only multipath propagated signal components) or as a fading Rice channel (the received signal also including a stable, or directly propagated, or strongly mirror-reflected part).
The multipath propagated signal components are of different phases in the receiver, since they have proceeded along different propagation paths. Such signal components of different phases are utilized in a RAKE receiver. When the signal components received by different branches are combined, the energy of the received signal is maximized. The RAKE receiver typically comprises several branches, whose delays are for example set to correspond with the delays of different signal components measured from the impulse response of the channel. Methods for setting delays for RAKE receivers are referred to as code phase acquisition or code acquisition methods and code tracking methods. Typically, a code is first searched for, and thereafter code tracking is carried out, where the delay values set during the search for code phase are specified. Patent publication FI982856, or patent publication WO00/41327, discloses a prior art code phase acquisition method of the RAKE receiver, which is incorporated here by reference.
RAKE receivers are for instance used in the UMTS system (Universal Mobile Telecommunications System), in which the frequency resource is divided into use by means of Code Division Multiple Access (CDMA), which is a broadband data transmission system. In a broadband system, the narrowband data signal of a user is modulated with a spreading code, which is more broadband than the data signal, on a fairly broad band. In the UMTS system, several users simultaneously send on the same frequency band and the data signals are separated from one another in the receivers based on a pseudorandom spreading code.
The spreading code is generally generated from a long preudorandom bit sequence. The bit rate of the spreading code is much higher than that of the data signal, and in order to distinguish the spreading code bits from the data bits and symbols they are referred to as chips. Each data symbol of a user is multiplied by spreading code chips. Thus, the narrowband data signal spreads over the frequency band used by the spreading code. The spreading code may include one or more data bits.
In the CDMA systems the RAKE receiver is synchronized with the spreading code sequence signal component specifically. Then, in order to perform code phase acquisition and code tracking the spreading code generator of the receiver is typically synchronized in accordance with the delay values obtained from the maximum points of the impulse response. A problem with the synchronizer, i.e. in setting the delays, is that code phase acquisition and code tracking require a lot of calculation capacity owing to the large number of samples. The impulse response also changes as a function of time. In addition, when the impulse response does not include distinct maximum points, but the impulse response comprises a broad power maximum region, in other words a fat finger situation, achieving the synchronization is difficult on the basis of an impulse response determined from a signal component received by one branch in accordance with the prior art and in general some of the energy of the received signal is lost.